Quantum computing is seen as the next big wave in computing technologies whereas classical computing, based on classical digital logic, is reaching its physical limitation. In order for universal quantum computing not only qubit (quantum bit) devices but also devices connecting the different qubits devices are required to allow a proper status detection. For this, ring resonators or circulators may be used. A circulator is a three (or more) port device with the unique characteristics that a specific port is unidirectional matched to its neighbor port on the left or right, but not to any other port (non-reciprocity). Today's common circulator technology is based on a magnetic field which typically is maintained by two permanent magnets.
The theory and implementation details can be reviewed in D. M. Pozar, +Microwave Engineering, (4th edition), John Wiley, 2005. The required macroscopic magnets prevent from integration of circulators into chip designs and from further down-scaling steps.
Related to superconducting quantum technologies, the magnets of traditional circulators have a significant negative influence on the neighboring circuitry and output signal measurements. Therefore, circulators usually need to be installed in a certain distance to the quantum circuits, which makes dense integration of these systems hard.
The paper of J. J. Lee et al., Magnetic Parameters for Ultra-high Frequency (UHF) Ferrite Circulator Design, Journal of Magnetics, Vol. 19, No. 4, pp 399-403, 2014, describes a design and fabrication of a ferrite based circulator and its measurements.
There are additional disclosures related to a superconducting circulator device. Document US 2015/0030280 A1 discloses a non-reciprocal device incorporating metamaterials which exhibit non-reciprocity through angular momentum biasing. The metamaterial, such as a ring resonator, is angular-momentum biased. This is achieved by applying a suitable mechanical or spatio-temporal modulation to resonant inclusions of the metamaterial, thereby producing strong non-reciprocity.
Document WO 2016/094045 A1 discloses a superconducting switch system that includes a filter network having an input portion and an output portion, and a variable inductance coupling element that couples the input portion to the output portion. The variable inductance coupling element has a first inductance that allows a desired portion of an input signal to pass from the input portion to the output portion as an output signal, and a second inductance state that suppresses the input signal from passing from the input portion to the output portion.
A disadvantage of known solutions is that often external magnetic fields are required or a solution is proposed comprising a varactor. However, this technology is not suitable for super conduction temperatures.
Hence, there may be a need to overcome the known disadvantages of today's common low temperature circulators and provide a circulator device not requiring external magnet fields or varactors.